Flow filter tubes are employed for the filtration of gas and liquid streams, and typically such tubes have been composed of a plurality of interrelated, nonwoven, randomly dispersed fibers which are bonded into a self-supporting flow filter tube at the junction of the fiber crossover points. The fibrous flow filter tube so formed is a self-supporting structure having the desired filtration efficiency and may be prepared so that compression across the full width of the tube edges provides for a fluid-tight seal, as set forth in Reissue Pat. No. 29,447.
Such a filter tube is prepared through, firstly, preparing an aqueous slurry of the fibers, and, thereafter, forming the slurry into a filter tube, typically with the aid of a vacuum mandrel to remove excess water and about which the filter tube is formed, and drying the vacuum-formed filter tube. The dried filter tube is then treated with a bonding agent, removed and, thereafter, heated to a temperature sufficient to effect the fusing or hardening of the bonding agent. The dry filter tube may be impregnated with the bonding agent by immersing the dry filter tube into a solution or emulsion containing the bonding agent, which is typically a resin, such as a thermosetting resin, such as an epoxy resin, phenol-formaldehyde resin or a silicone resin. Thereafter, the tube is heated to drive off any organic solvents in the solution used, or to effect the cross-linking under condensation of resin. To avoid the use of volatile solvents, the use of aqueous dispersions and emulsions of resins or of inorganic sols, such as silica sol, also have been employed as bonding agents.
The process of preparing glass-fiber filter tubes, for example, is set forth more particularly in U.S. Pat. No. 3,972,694, which relates to the use of a hardened silicone resin as a bonding resin. One process of forming a flow filter tube, containing a series of intermittent layers, is set forth, for example, in U.S. Pat. Nos. 4,124,360 and 4,078,956. The method of preparing such filter tubes and the filter tubes, themselves, also may have a series of internal scrim-like or porous support sheets formed within the internal wall of the filter tube or as an internal or external support, such as, for example, as set forth in U.S. Pat. No. 4,102,785.
It is desirable to prepare nonwoven, fibrous, self-supporting filter tubes at a lower cost and with better physical properties than those heretofore prepared which employ bonding agents. It also would be desirable to avoid the additional manufacturing steps occasioned through the use of a bonding agent and problems associated therewith.